usnic: convert from verbs to libfabric (yay!)

This commit represents the conversion of the usnic BTL from verbs to
libfabric.

For the moment, libfabric is embedded in Open MPI (currently in the
usnic BTL).  This is because the libfabric API is still changing, and
also has not yet been released.  Ultimately, this embedded copy of
libfabric will likely disappear and the usnic BTL will rely on an
external installation of libfabric.

New configure options:

* --with-libfabric: will cause configure to fail if libfabric support
    cannot be built
* --without-libfabric: will prevent libfabric support from being built
* --with-libfabric=DIR: use an external libfabric installation
* --with-libfabric-libdir=LIBDIR: when paired with --with-libfabric=DIR,
    use LIBDIR for the libfabric installation library dir

The --with-libnl3[-libdir] arguments are now gone.
common verbs: remove usnic-specific code

Now that the usnic BTL uses libfabric, we can remove the
usnic-specific code from opal/mca/common/verbs.
.gitignore: ignore usnic unit test executable
libfabric: sync to upstream libfabric github

Bring down the latest from the libfabric github, as of
9d051567c8eb7adc2af89516f94c7d0539152948.
usnic: update to latest libfabric API changes